Polypropylene is most often produced as a stereospecific polymer. Stereospecific polymers are polymers that have a defined arrangement of molecules in space. Both isotactic and syndiotactic propylene polymers, for example, are stereospecific. Isotactic polypropylene is characterized by having all the pendant methyl groups oriented either above or below the polymer chain. Isotactic polypropylene can be illustrated by the following general structural formula:
This structure provides a highly crystalline polymer molecule. Using the Fisher projection formula, the stereochemical sequence of isotactic polypropylene, as shown by Formula (2), is described as follows:
Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each “m” representing a “meso” dyad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the polymer chain lie on alternate sides of the plane of the polymer. Syndiotactic polypropylene can be illustrated by the following general structural formula:
Using the Fisher projection formula, the structure of syndiotactic polypropylene can be shown as follows:
The corresponding syndiotactic pentad is . . . rrrr . . . , with each “r” representing a “racemic” dyad. Syndiotactic polymers, like the isotactic polymers, are semi-crystalline. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer, which is non-crystalline and highly soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.
Although isotactic and syndiotactic polypropylene are both semi-crystalline polymers, they each have different characteristics or properties. Isotactic polypropylene has good processability during extrusion and molding or thermoforming. Isotactic polypropylene has a “hazy” or “cloudy” appearance that is undesirable for some applications. In contrast, syndiotactic polypropylene has excellent transparency or clarity and does not exhibit the haziness or cloudiness associated with isotactic polypropylene. Syndiotactic polypropylene also has good impact characteristics that are in many cases superior to isotactic polypropylene. Syndiotactic polypropylene is difficult to produce in commercially viable sheet form, however, due to its low melting point, slow solidification rate and narrow molecular weight distribution. As a result, the syndiotactic polypropylene is much softer and may exhibit surface tackiness when heated, which is undesirable and makes subsequent molding operations difficult. This is particularly undesirable in thermoforming applications, where clarity and impact strength of the product are important, but where the slow solidification rate and narrow molecular weight distribution of the polymer result in processing difficulties.
What is therefore needed is a polypropylene, particularly for use in thermoforming applications, wherein the polypropylene exhibits those desirable characteristics associated with syndiotactic polypropylene, but where the difficulties in processing and those undesirable characteristics often associated with syndiotactic polypropylene are reduced or eliminated.